if( document.documentElement.clientWidth >= 750 ){
  document.documentElement.style.fontSize = '100px';
}else{
  document.documentElement.style.fontSize = 100 * (document.documentElement.clientWidth / 750) + 'px';
}
 
setRem(document, window)
 
function setRem(doc, win){
  var docEl = doc.documentElement,
    resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
    recalc = function () {
        var clientWidth = docEl.clientWidth;
        if (!clientWidth) return;
        if(clientWidth >= 750){
            docEl.style.fontSize = '100px';
        }else{
            docEl.style.fontSize = 100 * (clientWidth / 750) + 'px';
        }
    };

  if (!doc.addEventListener) return;
  win.addEventListener(resizeEvt, recalc, false);
  doc.addEventListener('DOMContentLoaded', recalc, false);
}